In the brakes of the graduated release type, the air distribution is controlled by three pressures:
-- that of the brake pipe, PA1 -- that of a constant-pressure reservoir known as the control reservoir, PA1 -- that of the brake cylinder.
However, in such brakes, the pressure in the auxiliary reservoir destined to supply the brake cylinder during braking acts on the distributor instead of the pressure of the brake pipe during braking so as to link the discharge of the brake cylinder to the pressure prevailing in the auxiliary reservoir itself. An inexhaustible brake is obtained, provided the constant-pressure or control reservoir does not sustain losses.
Although such brakes may provide ideal safety on steep inclines, since they permit long and steep descents with total safety and good maneuverability, they have a serious drawback for flat traveling: Considerable time is needed for a brake release in a long train because the discharge of the cylinder itself is linked to the filling of the auxiliary reservoir, and the slow rate at which the auxiliary reservoirs on the vehicles relatively distant from the locomotive are recharged is translated into a correspondingly slow rate of discharging of the brake cylinder.
Notable progress has been made in recent decades in accelerating the brake release process with this type of distributor by introducing into such arrangements devices capable of accelerating refill of the auxiliary reservoirs at the rear of the train by reducing the amount of air absorbed from the brake pipe by the reservoirs at the head of the train, and by means of a prolonged refill of the system to the pressure of the main reservoir without the risk of overloading the control reservoirs of the first vehicles and the concomitant possibility of undesired rebraking or difficulties in brake release.
However, although the brake release times thus obtained are notably shorter than those obtained in the past with graduated release distributors of the same type, they are still longer than desirable, particularly for level traveling.